The present invention relates to an apparatus for recording and reproducing signals on a magnetic tape which is capable of reproducing signals at variable speeds.
A helical-scan video tape recorder (hereinafter simply referred to as a helical VTR or more simply VTR) is provided with a rotary magnetic head which scans tracks formed on a magnetic tape oblique to its direction of feed for recording and reproducing video and audio signals on the magnetic tape. In such helical VTR's, head tracking techniques allow the rotary magnetic head to follow the tracks on the tape so that high-quality reproduction can be achieved even if the tape is running at a speed different from that used during recording. In practice, in a broadcasting station, a helical VTR is set to single-frame reproduction mode at the starting point of news material or commercial (CM) material previously confirmed by single-frame reproduction, and the VTR is then switched to constant-speed reproduction mode at a desired time to send the news or CM material to a video screen device, etc. In this case, it is necessary to synchronize the color frame of the reproduced color video signal of the VTR with the color frame of a reference color video signal of, e.g., a parent VTR within a broadcasting station as quickly as possible to maintain the continuity and synchronization of the color subcarrier wave when the video signals are exchanged. This is because in NTSC (National Television System Committee) color television, interleaving has been adopted and so the frequency of the color subcarrier wave is an odd multiple of 1/2 of the horizontal scanning frequency f.sub.H so that both color subcarrier waves come into phase with each other at intervals of four fields.
VTR systems such as is shown in FIG. 1 have been used to carry out the color framing described above. In FIG. 1, numeral 1 denotes a rotary magnetic head, the output signal of which, i.e., the reproduced RF (radio frequency) signal is sent to an FM demodulator 3 via a reproduction amplifier 2. The color video signal outputted by the demodulator 3 is sent to a video output terminal 4. Numeral 5 denotes a capstan which controls the travel of the tape T and which is directly linked with a motor 6. Numeral 7A denotes a drive amplifier and 7C denotes a servo circuit. A reproduction control signal CTL from a control signal head (not shown) is sent to the servo circuit 7C via a terminal 8. A reference video siganal REF is sent to a pulse generator 9 via a signal terminal 10. The rotation of the capstan motor 6 is controlled by the output of the drive amplifier 7A which is the amplified output of the servo circuit 7C (phase error signal S.sub.E) derived from a reference phase signal S.sub.R from the pulse generator 9 and the control signal CTL received via the terminal 8.
Numerals 11 and 12 denote color framing detectors for the reproduced color video signals and reference color video signals REF respectively. The detection outputs of both detectors 11 and 12 are sent to a comparator 13 and the output signal of the comparator 13 is sent to the servo circuit 7C.
When the VTR shown in FIG. 1 changes from single-frame reproduction mode to constant-speed reproduction mode and the tape T begins to run, the servo circuit 7C receives the reproduction control signal CTL of the secondary tape from the terminal 8 and the reference phase signal S.sub.R, derived from the reference (parent) video signal REF shown in FIG. 2A, which goes "High" every other field as shown in FIG. 2B. The capstan motor 6 is controlled by the servo circuit 7C so that the reproduced control signal CTL is synchronized in phase with the reference phase signal S.sub.R. The secondary tape T enters the constant-speed running state in which the reproduced video signal is synchronized in phase with the reference video signal after the expiration of a draw-in time needed by the capstan servo system, which includes the motor 6, drive amplifier 7A, and servo circuit 7C (the draw-in time refers to the period of, e.g., one to several seconds required for the above-described phase synchronization).
As described above, the capstan servo system, i.e., the tape T is synchronized in phase (locked) with the reference video signal REF so as to make the color framing of the reproduced video signal and of the reference video signal REF coincide. Each group of four tracks TKa, TKb, TKc and TKd on the helical VTR tape correspond to first through fourth field intervals of a single color frame interval, as shown in FIG. 3. Control signal pulses CPa and CPc are recorded on the control signal track CTK at the start of the tracks TKa and TKc, that is, at the start of every other track.
In this way, since the control signal pulses (CPa etc.) are not recorded at the rate of one per four tracks of each color frame but rather at a rate of one track per two tracks of every frame, the tape T is synchronized in phase with the reference video signal REF. In the first field interval following activation of the secondary tape T, the track TKa corresponding to the first field of the color frame of the reproduced video signal and the track TKc corresponding to the third field will both be tracked by the reproduction rotary magnetic head 1 at probabilities of 50 percent. It should be noted that, in the case when the latter track TKc is the first to be tracked, the color framing of the reproduced video signal will fail to match that of the reference video signal REF.
In the case described above, the previously proposed VTR temporarily stops the servo circuit (7C) when the comparator 13 determines that the color framings do not coincide and thus the phase synchronization of the capstan servo system and the reference video signal REF is released. Under the above-described conditions, while the tape T is running in the direction denoted by the arrow V.sub.TP in FIG. 3, at the control signal pulse CPc of the third field, the phase of the capstan 5 is offset by two fields (=1 frame) from the control signal pulse of either CPa of the first field of the same subsequent color frame or from CPe of the first field of the next color frame.
As described above, in the previously proposed helical VTR, after the capstan servo system has once been synchronized with the reference video signal upon activation, it may be necessary for the capstan servo system to again be brought into phase with the reference video signal REF by offsetting the phase of the capstan by one frame in order to make the color framing of reproduced video signal coincide with that of the reference video signal REF.
Therefore, a redrawing-in time may also be needed in addition to the drawing time of the capstan servo system after the VTR is activated. In addition, since the phase of the capstan servo system will be incorrect during the redrawing-in time, there will be significant audio distortion in the reproduced signals.